Microelectronic devices typically comprise several dielectric, conductive, and semi-conductive layers. The operating characteristics of microelectronic devices are defined, at least in part, by the various layers in a device. A field effect transistor, for example, includes a doped silicon channel region under a layer of silicon dioxide, and the configuration of these layers can influence the performance as follows: arranging the channel region directly below the oxide layer mitigates parasitic capacitances; patterning the channel region across a width-wide axis of the oxide layer limits a maximum amount of channel current; and thinning the oxide layer decreases transistor turn-on voltage.
Variations in the properties of the individual layers will likewise create deviations in device performance. To ensure that a device has desired operating characteristics, microelectronic device fabrication requires stringent monitoring of the layers before, during, and/or after the processing steps. Such monitoring identifies layers with substandard properties and allows deviations in process steps to be quickly detected and corrected.
To accurately monitor device wafers, most fabrication facilities have a variety of metrology tools that are dedicated to inspecting key properties of the layers and/or features on a wafer. For example, particle counters reveal how many particulates have been deposited or otherwise formed on a layer; four-point probes measure the resistance of doped silicon and deposited metal layers; and inline scanning electron microscopes (SEMs) facilitate inspection of submicron layer geometries.
One property that is closely monitored is layer thickness. To monitor layer thickness, most facilities include an ellipsometer in their repertoire of metrology tools. An ellipsometer detects layer thickness by reflecting polarized light off of a surface of the wafer and analyzing the incident light. In addition to layer thickness, ellipsometers also provide information about layer uniformity and output relative thicknesses on a wafer map. Because ellipsometers employ a light beam, the detection is non-contact and non-destructive. In addition to being non-contact, ellipsometry is generally highly automated. Typically, an ellipsometer includes a user-programmed wafer map, which guides the ellipsometer to specific detection points on the wafer. Thus ellipsometers are also generally high throughput tools.
As device geometries become smaller, ellipsometry tools need to be able to measure smaller layer thicknesses. A well calibrated ellipsometer, for example, can detect thickness variations in the gate oxide of newer generation field effect transistors having thicknesses on the order of tens of angstroms. Furthermore, in smaller device geometries, the layer topography can also provide useful information for assessing the properties of a layer or other structure. For example, in older generation transistors having thick metal interconnects, small topography variations in the underlying isolation oxide may not translate into a significant metal-line surface height variation. However, as metal lines become thinner, small topography variations in the underlying oxide can produce sizable metal-line surface height variations. These topography variations can cause incomplete metal coverage, increased interconnect resistance, or localized electrical fields, all of which may ultimately result in interconnect failure.